Differential relays operate on three or more sets of three-phase current inputs. These relays can be older electromechanical relays with current transformers (CTs) connected in parallel to the three-phase current inputs or newer numerical relays that have more than two sets of three-phase current inputs. Note that it is poor practice to connect CTs in parallel to a single three-phase current input; if one CT should saturate during a condition such as transformer inrush or an external fault, all the other CTs will saturate as well since they have the same voltage drop across their secondary. The relay is in turn much more likely to misoperate since it sees the sum total excitation current as differential current.
Figure 1 is a simple one-line diagram illustrating the differential circuit for a two-winding transformer (T1) protected by four sets of CTs. The polarity for each individual CT is not shown as they may not all be connected in the same exact convention; however, doing so is the best practice. If the CT polarities are not consistent, testing the differential protection can be much more difficult, and unwanted operations are more likely to occur because of unfound errors.
Figure 2 shows the current flow for both standard and non-standard CT polarities. Current flows into the primary polarity and out of the secondary polarity as illustrated at the top. The relay current input is connected in reverse as illustrated at the bottom, which is another poor practice.
Non-Standard CT Connections
Figure 3 shows the current flow for one branch of a differential relay three-phase current input. Current is flowing out of the zone of protection (e.g., transformer), which is above the dashed line. Secondary current flows into the non-polarity terminal of the differential relay current inputs (shown for A-Phase) during normal load flow and external faults located below the dashed line. This is also poor practice and even more so when all of the CTs for one differential relay are not connected the same.
Standard CT Connection
Figure 4 shows the standard and correct CT connection for differential protection. Note the polarity marks are away from the zone of protection. Secondary current still flows into the non-polarity terminal of the differential relay current inputs (shown for A-phase) during normal load flow and external faults located below the dashed line.
This article illustrates the best connections for CT polarity when providing differential protection. Typically, all CTs should be connected the same, and the polarity marks should face away from the zone of protection.
Steve Turner is in charge of system protection for the fossil generation department at Arizona Public Service Company in Phoenix. After working with Beckwith Electric Company, Inc. for 10 years, Steve spent two years as a consultant in San Diego. His previous experience includes positions as an Application Engineer at GEC Alstom and in the international market for SEL focusing on transmission line protection applications. Steve also worked for Duke Energy (formerly Progress Energy), where he developed the first patent for double-ended fault location on overhead high-voltage transmission lines and was in charge of all maintenance standards in the transmission department for protective relaying. Steve has BSEE and MSEE degrees from Virginia Tech University. He has presented at numerous conferences including Georgia Tech Protective Relay Conference, Western Protective Relay Conference, ECNE, and Doble User Groups, as well as various international conferences. Steve is a senior member of IEEE and a member of the IEEE PSRC.